High fructose syrups are commercially manufactured by enzymatically isomerizing dextrose syrups to a fructose containing syrup with glucose isomerase. Illustrative strains which reportedly possess commercial potential include Actinoplanes missouriensis, Arthrobacter sp., Bacillus coagulans, Streptomyces albus, S. phaeochromogenes, S. olivaceus, S. olivochromogenes, and S. wedmorensis.
Streptomycetes typically require relatively neutral culture media pH (e.g., &gt;pH 6.0) for growth and glucose isomerase production. The above Streptomyces species cannot effectively grow and produce glucose isomerase at a pH less than 5.5.
Streptomycetes capable of growing under acidic conditions have been sparsely reported. Currently, these atypical Streptomyces species are regarded as a laboratory curiosity. An early report on an Actinomyces acidophilus (subsequently designated as Streptomyces acidophilus), which could be cultivated under acidic conditions (e.g., pH 4.0), had been isolated from Danish soil by Jensen (see Soil Sci. 25: 225-234, 1930). This streptomyces has been lost (e.g., see Bergey's Manual of Determinative Bacteriology--8th Ed., R. E. Buchanan and N. E. Gibbons co-editors--"no reference strains known"). Alexander (Introduction to Soil Microbiology, John Wiley & Sons, Inc., 1967) reports streptomycetes in slightly acidic environments will comprise less than 1% of the total viable bacterial count and they are essentially extinct in soils having a pH 5.0 or less. Recently Hagedorn (Appl. Environ. Microbiol. 32: 368-375, 1976) reported the isolation of acidophilic, acidoduric, and neutrophilic streptomycetes strains from acidic forest soils. The art failed to recognize that this atypical class of microorganisms produced commercially valuable enzymes. The difficulty in isolating these acidic streptomycetes (e.g., rare occurrence; presence of other predominant microorganisms; pH of culture media, etc.) may partially explain this lack of scientific interest and appreciation.
The inventors wished to explore the feasibility of obtaining valuable enzymes from these atypical Streptomyces. The microorganisms were unobtainable from public culture collections and depositories. Accordingly, wild-type Streptomyces for study purposes had to be isolated from a suitable source. During the course of such experimentation, it was unexpectedly discovered that these acid-loving Streptomyces elaborated glucose isomerase. By selective mutation, Streptomyces strains possessing significantly improved growth and glucose isomerase elaboration characteristics were subsequently discovered.